1. Field
This invention relates generally to memory circuits, and more specifically to a sense amplifier for use with memory such as flash memory.
2. Related Art
Flash memory stores data in one or more non-volatile memory devices, each of which comprises a floating gate n-channel metal-oxide-semiconductor field-effect transistor (hereinafter “bitcell”). A bitcell stores one bit of information. The bitcell maintains data even if the bitcell is disconnected from a power supply.
Data storage is based on a threshold voltage of the bitcell, which depends on an electric charge present in a floating gate. If the floating gate has a lack of electrons, it becomes positively charged. Such positive charge of the floating gate attracts electrons to a surface of a bulk; therefore, a control gate needs less voltage to attract electrons to the surface of the bulk. Thus, the threshold voltage of the bitcell decreases. On the other hand, if the floating gate is negatively charged, it repels electrons from the bulk; therefore, fewer electrons are available at the surface of the bulk. In this situation, the control gate needs more voltage to attract electrons from the surface of the bulk. Thus, the threshold voltage of the bitcell increases.
A logic level of a bit stored in a bitcell can be determined by measuring a drain current of the bitcell that occurs as a result of a read voltage being applied to a gate of the bitcell during a read operation. A sense amplifier is a circuit that detects an amount of the drain current of the bitcell, and, based on the amount of the drain current, outputs a signal that is indicative of the logic level of the bit stored in the bitcell.